**4.2 Experimental section for VOCs breath analysis**

Exhaled breath was analyzed using a LPAS system and we have measured ammonia and ethylene concentration from the exhaled breath in subjects with T2DM and healthy subjects [66, 75, 76]. The block diagram of the laser PA spectrometer is presented in **Figure 2**.

LPAS main components of the system are: a CO2 laser (home-built) emitting in the 9.2–10.8 μm range (area where ammonia and ethylene shows a high

**155**

formula:

**Figure 2.** *CO2LPAS system.*

Where:

**4.3 Breath collection**

*Organic Volatile Compounds Used in Type 2 Diabetes DOI: http://dx.doi.org/10.5772/intechopen.94752*

absorption), frequency-stabilized and an output power between 2 and 5 W, and an external PA cell (the external resonator home-build), inside it being mounted four microphones (sensitivity of 20 mV/Pa each), connected in series are mounted flush with the internal surface of the resonator tube. Before entering into the PA cell, the cw laser beam is modulated by a mechanical chopper that operate at the resonant frequency of the PA cell (564 Hz) and focused by a ZnSe lens. The laser beam power after passing through the PA cell is measured by a radiometer connected to a data acquisition interface module together with a lock-in amplifier. The acquisition interface is connected to a computer where all experimental data are processed in real-time and stored. The software allows the display of several parameters, such the values for the PA voltage, average laser power after chopper, and the trace gas concentration, and the response of the PA system is based on the

*V = αCPLSMc.*

*V* (V)—photoacoustic signal (peak-to-peak value);

*C* (Pa cm W−1)—cell constant;

*P*L (W)—cw laser power before chopper; *S*M (V Pa−1)— microphone responsivity;

without being necessary any disconnections.

*α* (cm−1 atm−1)—gas absorption coefficient at a given wavelength;

Another important part of the CO2LPAS system is represented by the gas handling system. It has the role to ensure the purity of the PA cell, to introduce the sample gas into the PA cell at a controlled flow rate, to pump out the sample gas from the PA cell, and to monitor the total and partial pressures of mixture gas sample. This several functions can be performed by the gas handling system

In breath analysis, depending on the desired result, is required a knowledge and understanding of respiratory physics. In the individual, normal, resting breathing is about 0.5 L, breathing known as tidal volume. The total volume of the lung is about

*c* (atm)—concentration or partial pressure of the trace gas.

**Figure 1.**

*Schematic of the physical processes occurring in PAS [76].*

*Organic Volatile Compounds Used in Type 2 Diabetes DOI: http://dx.doi.org/10.5772/intechopen.94752*

**Figure 2.** *CO2LPAS system.*

*Type 2 Diabetes - From Pathophysiology to Cyber Systems*

rapid to allow the simultaneous analyses of several trace gas metabolites in single breath exhalations. Over the years, the LPAS technique has demonstrated its ability to detect traces of gas in fields such as biology and medicine due to several factors, such as: real-time detection of one or more volatile compounds, detection limits ranging from ppm (parts-per-million) to ppb (parts-per-billion), high sensitivity and selectivity, use of a single breath collection from a small sampling volume (few

Laser photoacoustic spectroscopy is based on the photoacoustic effect that occurs at the interaction between light and matter with the generation of a sound wave. In 1880, Alexander Graham Bell discovered these phenomena [83] while trying to find wireless communication. Thus, he discovered that certain optically absorbing solids emit a sound when illuminated by a modulated light. In 1881, Bell [84] Tyndall [85], Röntgen [86] and Preece [87] have demonstrated that the photoacoustic (PA) effect occurs not only in solid but also in liquid and gas. They found also, that the sound was stronger when the sample was placed in a cavity called photophone or spectrophone. With the appearance of sensitive microphones, increased interest in this technique. Afterwards, techniques based on this phenomenon have been known a continuous development, and today can be applied in

An instrument based on the PA effect and which uses a laser as a radiation source, has important advantages for the analysis of gas traces such as high sensitivity ppb or even ppt (parts per trillion) concentrations and selectivity, high dynamic range, high accuracy and precision, good time resolution, versatility, reliability, robustness and

Over the years, photoacoustic spectroscopy (PAS) has proven its ability to detect traces of gas and has been used successfully as a gas sensor in biological and medical

In gases, the PA effect is produced as a result of the following sequences (see **Figure 1**) [76]: absorption of incident laser radiation modulated in frequency or amplitude by the target gas molecules; local heating due to non-radiative relaxation; the extension and contraction of the gas sample that determines the pressure variation, which is an acoustic wave; detection of acoustic waves using microphones.

Exhaled breath was analyzed using a LPAS system and we have measured ammonia and ethylene concentration from the exhaled breath in subjects with T2DM and healthy subjects [66, 75, 76]. The block diagram of the laser PA spec-

LPAS main components of the system are: a CO2 laser (home-built) emitting in the 9.2–10.8 μm range (area where ammonia and ethylene shows a high

100 ml) without the need for further preparation [66–68, 75–82].

**4.1 Laser photoacoustic spectroscopy: basic principles**

almost all disciplines of Science and Technology.

**4.2 Experimental section for VOCs breath analysis**

trometer is presented in **Figure 2**.

*Schematic of the physical processes occurring in PAS [76].*

is easy to use.

applications [88–93].

**154**

**Figure 1.**

absorption), frequency-stabilized and an output power between 2 and 5 W, and an external PA cell (the external resonator home-build), inside it being mounted four microphones (sensitivity of 20 mV/Pa each), connected in series are mounted flush with the internal surface of the resonator tube. Before entering into the PA cell, the cw laser beam is modulated by a mechanical chopper that operate at the resonant frequency of the PA cell (564 Hz) and focused by a ZnSe lens. The laser beam power after passing through the PA cell is measured by a radiometer connected to a data acquisition interface module together with a lock-in amplifier. The acquisition interface is connected to a computer where all experimental data are processed in real-time and stored. The software allows the display of several parameters, such the values for the PA voltage, average laser power after chopper, and the trace gas concentration, and the response of the PA system is based on the formula:

$$V = a \mathbf{C} P\_L \mathbf{S}\_M \mathbf{c}.$$

Where:

*V* (V)—photoacoustic signal (peak-to-peak value); *α* (cm−1 atm−1)—gas absorption coefficient at a given wavelength; *C* (Pa cm W−1)—cell constant; *P*L (W)—cw laser power before chopper; *S*M (V Pa−1)— microphone responsivity; *c* (atm)—concentration or partial pressure of the trace gas. Another important part of the CO2LPAS system is represented by the gas

handling system. It has the role to ensure the purity of the PA cell, to introduce the sample gas into the PA cell at a controlled flow rate, to pump out the sample gas from the PA cell, and to monitor the total and partial pressures of mixture gas sample. This several functions can be performed by the gas handling system without being necessary any disconnections.
